A typical procedure in which semiconductor devices are fabricated from wafers entails heating the wafers during several process steps. In contemporary practice, wafers are routinely heated with resistance furnaces, infrared or quartz-halogen lamps, electron beams, and lasers. In some applications, radio frequency energy is used to heat a susceptor from which the thermal energy is transferred to a wafer by conduction, convection, or radiation.
The principal problem associated with the utilization of an apparatus for microwave heating of low-resistivity semiconductor ribbons and wafers without a susceptor is the creation of an efficient applicator, i.e., the device with which the microwave energy is applied to the sample to be heated. Previous attempts to heat ribbons and wafers in this manner failed to provide either efficient coupling of the microwave energy into the sample or uniformity of heating.
The present invention relates to innovative means and methods of applying traveling microwaves to thin low resistive semiconductor pieces and, more particularly, to means and methods of heating semiconductor ribbons or wafers directly by microwave energy without using a susceptor. The elimination of the susceptor is highly desirable because it obviates the need for efficient heat transfer between the susceptor and the wafer and eliminates the possibility of wafer contamination by the hot susceptor. The realization of this goal thus provides important and unique means and methods for the diffusion, drying, sintering and rapid annealing of such wafers and ribbons which means and methods are both convenient and cost effective.
Prior attempts, albeit less than highly successful, have been described in the literature for somewhat similar problems. For example, Guidici (Siltec Corporation, Menlo Park, CA.) described experiments for producing photovoltaic devices in which coin-stacked wafers were placed in a microwave applicator and heated to 900.degree. C. The absorption of microwave radiation near the exterior surfaces of the stack generated heat which was transmitted to the interior of the stack by thermal conduction. Guidici has used the same apparatus for sintering metallization coatings on single wafers.
Other experiments in which microwave energy was used to heat a small silicon sample were recently described by Chenevier et al at CNRS in Grenoble. (See: Pulsed annealing of semiconductors by microwave energy, Chenevier et al J. Physique-LETTERS, 43 (1982) L-291-294). The principal feature of the CNRS method was the use of the small silicon sample as part of the wall of a standing-wave resonator made from x-band waveguide. When the resonator is excited by the microwave field, the wall currents resulting from the microwave field heat the sample because of its non-zero resistivity. The procedure is alleged to be energy efficient (up to 30% is claimed), and the apparatus required to implement it is quite conventional. To facilitate absorption of microwave energy by a cool sample of relatively high resistivity, Chenevier et al use an incandescent lamp to decrease the resistivity of the sample by photoexcitation of carriers. This procedure is, however, suitable only for small samples as both thermal and electrical problems occur at the sample edges.
It is apparent that a clear and present need still exists for the development of means and methods of applying microwaves to heat thin low-resistivity semi-conductor ribbons and wafers without requiring the use of a susceptor therewith. It is toward this need that the present invention is directed.
Accordingly, a principal object of the present invention is to provide new and improved means and methods for heating low-resistivity materials, such as semiconductor materials with microwaves without a susceptor whereby the material being heated is the hottest body within the applicator and the possibility of contaminating the sample by a susceptor is eliminated.
Another object of the present invention is to provide a new and improved method of heating semiconductor materials which has a relatively short process time because the microwave energy is dissipated directly into the semiconductor samples rather than to and through a susceptor.
A further object of the present invention is to provide new and improved means and methods for heating low-resistivity materials such as semiconductor materials with microwaves which have substantially enhanced energy efficiency.
These and still further objects as shall hereinafter appear are readily fulfilled by the present invention in a remarkably unexpected manner as will be readily discerned from the following detailed description of an exemplary embodiment thereof especially when read in conjunction with the accompanying drawing in which like parts bear like numerals throughout the several views.